In a typical hard disk drive, digital data is written onto magnetic media by changing the polarity of a write current, Iw, which is applied to an inductive write head as the head travels over the surface of the selected magnetic media. The write current produced and switched by the write circuit associated with a particular write head produces its write current in current pulses of selected amplitude having either a positive or negative fixed magnitude. In particular, the write circuit switches fixed magnitude current pulses between two oppositely directed yet equal magnitude values, i.e., +Iw and -Iw. The speed of current switching between these two values follows the relationship V=L(di/dt). This can be rewritten as follows: EQU di/dt=V/L equation (1)
where di/dt is the change in current over time,
V is the voltage swing across the write head and PA1 L is the inductance of the write head.
According to this relationship, the voltage swing across the head is critical to high speed current switching through the write head. Conventionally, write circuits include an H-type current switch including four transistors, a write current source, and an inductive write head. The H-type switch has two states, in which a particular pair of the transistors is on and the remaining pair of transistors is off (state one) or which the remaining pair of the transistors is on and the particular pair of transistors is off (state two). The two states differ in the direction of the write current flowing through the write head. The direction of current flow is controlled by data signals.